You would be pushing your PSU a bit too hard. Don't forget to account for CPU power consumption as well as other devices in the PC. I would recommend at the very least a 600W, but you can pick up a 750W for fairly cheap now, giving you a little room to upgrade your gfx in the future.

It can deliver 500W yes, but with ripple and noise way outside of the ATX standard. I wouldn't risk my hardware like that.

This has nothing to do with efficiency or if the PSU can deliver enough power. The dangerously high ripple and noise values can destroy the components.

If the ripple and noise is really high, it will do more than just corrupt data. It can shorten the life of attached hardware, or even fry it outright. I've seen several PCs destroyed when the PSU's capacitors went bad, which makes the ripple go through the roof.

so then should i be better off buying a single card
such as 7870 or gtx 660?

Yes. Buying two weak cards for Crossfire or SLI will always end in tears. Dual card gaming has so many problems you should never do it unless you actually need the power of two high end cards to run triple screen gaming monsters.

Never going to log into this garbage forum again as long as calling obvious troll obvious troll is the easiest way to get banned.
Trolling should be.

Yes. Buying two weak cards for Crossfire or SLI will always end in tears. Dual card gaming has so many problems you should never do it unless you actually need the power of two high end cards to run triple screen gaming monsters.

Most the power consumption there comes from the 12V rail. It's pushing it with a 500W low quality PSU and I would NOT recommend it under any circumstance. 400-500W PSU is enough to run most systems with 1 video card. When we're talking two video cards we're talking 600W+ PSU's based on the quality of the manufacturer. For a "lesser brand" I would recommend adding 50% wattage above what you need in order to account for noice, ripple and generally low wattage out from the 12V rails.

TL;DR
Don't even think about it. Worst case scenario you will not only damage your PSU but also the rest of your PC. Get a new PSU or a powerful single card solution. The latter is the best option anyway unless you want to live in Crossfire's microstutter heaven. 7970GHZ will outperform those 7850's in actual game performance anyway.

Watts is almost irrelevant, but yes 500 watts is more than enough assuming you just have a HDD and no extra lights and shit hooked up.

I have no idea why manufacturers rate PSU's in watts when AMPS is the far more important factor. You are going to want around 45 amps for crossfire 7850's.

But like other people have stated, get crossfire out of your head, unless you want more problems than it is worth. Sure its cool looking and crossfire is a sexy term, but there are games out there that play worse with crossfire than with a single card. Crossfire also does not work with windowed mode, meaning if you play a game that supports windowed fullscreen (the best mode) you wont be able to take advantage of it because of crossfire.

I have no idea why manufacturers rate PSU's in watts when AMPS is the far more important factor.

The wattage of the unit is from the maximum load possible. This includes the 3.3V and 5V rails as well as the 12V rail(s).

So they provide the 3.3 and 5V rails with lots of amps (maybe 30-40 each (99-132 and 150-200 respectively) and the 12V rail gets maybe 15W (180W) on a "500W power supply".

So they can advertise a crappy unit as a good one, so long as the customer doesn't know to look at the label. And lets face it, put yourself in the shoes of somebody with a budget and no knowledge of computer hardware. They see a "500W PSU" for 25 bucks, and a "500W PSU" for 80 bucks. Which would they buy?